1. Field of the Invention
The present invention relates to a color liquid crystal display device with a color filter for the primary colors.
2. Description of the Related Art
Color liquid crystal display devices are used for displaying a television picture image. This type of display device comprises two substrates, one having a plurality of transparent scanning electrodes formed thereon and the other having a plurality of signal electrodes formed thereon and facing the scanning electrodes, a liquid crystal provided between these substrates, a pair of polarizing plates disposed outside the substrates and a color filter colored in the primary colors of red, green and blue in association with the scanning electrodes. Each cross point between the scanning and signal electrodes constitutes a pixel and three adjacent pixels constitute a single element for a color image.
The color filter for use in the above display device is formed on the transparent signal electrodes formed on the second substrate. This color filter is formed by coloring a staining base material, made of a protein such as casein or gelatin, with a dye. The base material of the color filter is a dielectric and the liquid crystal is disposed between the color filter and the scanning electrodes. Therefore, the liquid crystal is applied only with a voltage which is attained by dividing the voltage applied between the signal and scanning electrodes in accordance with the dielectric constants of the color filter (dielectric) and the liquid crystal. Accordingly, the conventional color liquid crystal display device uses a liquid crystal with a low response characteristic or requires a high driving voltage.
To solve the above problem, there has been proposed a color liquid crystal display device which has electrodes formed on the color filter so that the liquid crystal can be applied with substantially the same voltage as applied between facing electrodes. The structure of such color liquid crystal display is disclosed in Japanese Patent Disclosure No. 60-159830, 60-159823 and 61-51126. According to the disclosed structure, a transparent conductive film is formed directly on a color filter. With this structure, however, a conductive inorganic material such as Indium-Tin-Oxide (hereinafter referred to as ITO) is formed on a color filter made of a colored organic material such as casein or gelatin, thus impairing the adhesiveness between the color filter and the conductive film. In addition, as the inorganic conductive film is formed by means of, for example, sputtering so that the surface temperature of the color filter is increased. This makes it difficult to form the transparent conductive film made of ITO or the like which has an excellent characteristic.
The solution to this problem is proposed which first forms a transparent insulative film on a color filter and then forms transparent electrodes thereon as disclosed in Japanese Patent Disclosure No. 61-43727. A Similar structure is disclosed in Japanese Patent Disclosure No. 60-159831 in which a transparent insulative film made of an organic material such as an acrylic resin is formed on a color filter and transparent electrodes made of ITO is then formed on the insulative film. Since the transparent insulative film made of the aforementioned organic material has a low acid resistance and has a water absorption, however, the color filter disposed below the insulative film is likely to be damaged by acid etching liquid and the dye in the color filter may leak out in the patterning or the like of the ITO conductive film formed on the insulative film. The color filter for use in the conventional color liquid crystal display device is therefore difficult to produce with a good characteristic.
Further, the color liquid crystal device requires a good color balance. With a color liquid crystal display device with pair of polarizing plates having their polarizing axes arranged in parallel to each other, however, if pixels of the individual colors are driven by the same voltage, the transmissivities of lights passing through the pixels of three different colors would differ from one another. This is because the optical anisotropy (.DELTA.n) for the lights of individual wavelengths differ from one another and the transmissivities is determined by .DELTA.n multiplied by the thickness of the liquid crystal layer, d, i.e., .DELTA.n.multidot.d. Consequently, the light passing through the liquid crystal device appears to be colored. That is, due to different transmissivities of lights of the different wavelengths which are to pass through the associated color filters, the entire screen of the display device appears to be in a specific color. For instance, according to a color liquid crystal display device of a simple matrix type, the transmissivities of red light is high, so that the entire screen looks reddish or provides a sepia color.
According to an active matrix type liquid crystal display device which has a active element such as a thin film transistor (TFT) coupled to each display electrode constituting each pixel, the thickness of the liquid crystal layer is varied for those portions of the color filter which correspond to the color filter colors in order to prevent the color imbalance. This structure is disclosed in, for example, Japanese Patent Disclosure No. 60-159823 60-159830 60-159827, 60-159831 61-98330 and 61-121033. The liquid crystal display device disclosed in these documents have different thicknesses for those corresponding to the R, G and B filters. The layer thickness is set in such a manner that the values of .DELTA.n.multidot.d at those portions where the R, G and B filters are formed coincide with .DELTA.n.multidot.d at the points (.alpha.1, .alpha.2, .alpha.3) where the transmissivities represented by the transmissivity curves B, G and R of lights for the individual colors first indicate the minimum values, as shown in FIG. 1 which illustrates transmissivities with respect to .DELTA.n.multidot.d for lights passing though the individual R, G and B filters. According to the active matrix type liquid crystal display device, therefore, the color balance is improved by varying the thickness of the liquid crystal layer at those portions which correspond to the individual color filters for the following reason. According to the active matrix type liquid crystal display device, since the liquid crystal is applied with a substantially static voltage, each pixel is turned off and on respectively by a voltage Va which provides the darkest state in FIG. 2 that illustrates the transmissivities with respect to a voltage and a voltage Vb which provides the brightest state, and in the OFF state, the values of .DELTA.n.multidot.d for the orientation of liquid crystal molecules substantially equal those of .DELTA.n.multidot.d for the initial orientation of these molecules.
According to the simple matrix type liquid crystal display device having mutually-facing electrodes respectively formed on two substrates, however, the individual pixels are driven in a multiplex manner so that a bias voltage is also applied between facing electrodes of the pixels in the OFF state. That is in FIG. 2, voltage Va1 is applied between these electrodes in the OFF state while voltage Vb1 is applied in the ON state. Consequently, the orientation of the liquid crystal varies by the bias voltage so that unlike in the case of the active matrix type, the color balance cannot be acquired by setting .DELTA.n.multidot.d at the initial state equal to the values .alpha.1, .alpha.2 and .alpha.3 in FIG. 1 for the pixels associated with the individual colors. According to the conventional simple matrix type liquid crystal display device, it is difficult to control the color balance. In this respect, therefore, .DELTA.n.multidot.d is set in the range between .beta.1 and .beta.2 as shown in FIG. 1, i.e., 1.1 and 1.2, in consideration of a relatively low wavelength-dependency of the transmissivity and a high contrast for a large value of .DELTA.n.multidot.d. Due to a relatively large value of .DELTA.n.multidot. d, however, the conventional liquid crystal display device has a narrower viewing angle and cannot have the color balance sufficiently well controlled.